Exposure control apparatus and method

ABSTRACT

An apparatus has a pulse light source for emitting light pulses with varying light quantities, an illumination optical system for radiating the light pulses from the source onto a predetermined illumination region on a mask on which a transfer pattern is formed, and a projection optical system for projecting an image of the pattern onto a predetermined exposure region on a substrate, and which synchronously scans the mask and the substrate during the projection. The apparatus includes a measuring device for detecting intensities of the light pulses radiated onto the substrate during the scanning and measuring an integrated light quantity on each of a plurality of partial regions in the exposure region on the substrate on the basis of a detection signal of the intensities, wherein the partial regions are defined by a scanning speed of the photosensitive substrate and an oscillation interval of the light pulses. The apparatus further includes an adjusting device for adjusting an intensity of the next light pulse to be radiated onto the mask on the basis of a difference between a target integrated light quantity and the measured integrated light quantity on each of the partial regions when some light pulses are radiated onto the mask.

This is a continuation of application Ser. No. 08/201,126 filed Feb. 24,1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure control apparatus andmethod for controlling the exposure value onto a photosensitivesubstrate and, more particularly, to exposure control of an exposureapparatus of a slit-scanning exposure type for exposing a pattern on amask onto a photosensitive substrate by illuminating a rectangular orarcuated illumination region with light pulses from a pulse lightsource, and synchronously scanning the mask and the photosensitivesubstrate with respect to the illumination region.

2. Related Background Art

Conventionally, in the manufacture of a semiconductor element, aliquid-crystal display element, a thin-film magnetic head, or the likeusing a photolithography technique, a projection exposure apparatus forexposing a pattern on a photomask or a reticle (to be referred to as a"reticle" hereinafter) onto a photosensitive substrate such as a waferor glass plate coated with, e.g., a photoresist via a projection opticalsystem is used. Recently, the size of a single chip pattern (one shotarea radiated onto a wafer) on a semiconductor element tends to becomelarge, and the projection exposure apparatus is required to expose apattern having a larger area on a reticle onto a photosensitivesubstrate (large area requirement). Also, it is required to increase theresolution of the projection optical system in correspondence with adecrease in line width of a pattern of, e.g., a semiconductor element.

However, it is not easy to increase the resolution of the projectionoptical system and to simultaneously increase the size of an exposurefield of the projection optical system. In particular, when acatadioptric system is used as the projection optical system, anaplanatic exposure field often has an arcuated shape.

In order to meet the above-mentioned large area requirement of a patternto be transferred and limitation on the exposure field of the projectionoptical system, a projection exposure apparatus of a slit-scanningexposure type has been developed. In this apparatus, by synchronouslyscanning a reticle and a photosensitive substrate with respect to, e.g.,a rectangular, arcuated, or hexagonal illumination region (to bereferred to as a "slit-shaped illumination region"), a pattern, havingan area wider than the slit-shaped illumination region, on the reticleis exposed onto the photosensitive substrate. In general, in aprojection exposure apparatus, since an appropriate exposure value for aphotosensitive material on a photosensitive substrate is determined, theprojection exposure apparatus of the slit-scanning exposure typecomprises an exposure control apparatus for controlling the exposurevalue with respect to the photosensitive substrate to coincide with anappropriate exposure value within a predetermined allowable range.

As one technique for increasing the resolution of a pattern to beexposed onto a photosensitive substrate, a technique for decreasing thewavelength of exposure light is known. In association with thistechnique, of existing light sources, those which emit light having ashort wavelength are pulse-oscillation type laser light sources(pulse-oscillation light sources) such as an excimer laser light source,a metal vapor laser light source, and the like. However, unlike acontinuous emission type light source such as a mercury lamp, energy oflight pulses emitted from a pulse-oscillation light source varies withina predetermined range in units of pulse emissions.

Therefore, in the conventional exposure control apparatus, when theaverage energy of light pulses emitted from the pulse-oscillation lightsource is represented by pa, and the range of a variation in pulseenergy of the light pulses is represented by Δp, it is assumed that aparameter Δp/pa representing the variation in pulse energy has a normaldistribution (is random). When the number of light pulses radiated ontoa certain region (to be referred to as a "pulse count integratingregion" hereinafter) on a photosensitive substrate which is scannedrelative to an exposure region conjugate with a slit-shaped illuminationregion illuminated with light pulses is represented by n, by utilizingthe fact that a variation in integrated exposure value after the end ofexposure is given by (Δp/pa)/n^(1/2), the integrated exposure value iscontrolled to reach an appropriate exposure value within a predeterminedallowable range under the assumption that the variation (Δp/pa) in pulseenergy does not exceed a predetermined value. For example, when Δp/pathree times a standard deviation σ is assumed to be 10%, in order to seta desired reproduction precision A of an integrated exposure value threetimes the standard deviation σ to be 1%, n is 100 or more. Therefore, itsuffices if the reticle and the photosensitive substrate aresynchronously scanned relative to a slit-shaped illumination region, sothat the number of light pulses radiated onto each pulse countintegrating region on the photosensitive substrate becomes 100 or more.

However, since conventional exposure value control is open control, whenthe oscillation state of the pulse-oscillation light source fluctuatesfor some reason, and the variation (Δp/pa) in pulse energy temporarilyexceeds 10%, the desired reproduction precision A of the integratedexposure value can no longer be obtained.

In order to solve this problem, in a projection exposure apparatus suchas a stepper for exposing a pattern on a reticle onto a photosensitivesubstrate while the reticle and the photosensitive substrate standstill, as disclosed in commonly assigned Japanese Laid-Open PatentApplication No. 63-316430 and U.S. Pat. No. 4,970,546, a modifiedexposure method for performing exposure by reducing some last lightpulses, and a cutoff method for ending exposure when the integratedexposure value reaches an appropriate exposure value within a targetprecision range are known. In the cutoff method, the number of lightpulses radiated onto the photosensitive substrate is not constant.Furthermore, as filed in commonly assigned U.S. patent application Ser.No. 623,176 (May 12, 1990), a technique for controlling an exposurevalue by finely adjusting pulse energy in units of pulses is also known.

However, due to the unique feature of the projection exposure apparatusof the slit-scanning exposure type, that is, since light pulses radiatedon a plurality of pulse count integrating regions on the photosensitivesubstrate have different integrated energy levels, the above-mentionedexposure value control method proposed for a non-scanning type exposureapparatus cannot be directly applied.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of theabove-mentioned problems, and has as its object to provide an exposurecontrol apparatus for an exposure apparatus which synchronously scans areticle (mask) R and a photosensitive substrate (W) relative to aslit-shaped illumination region by illuminating the slit-shapedillumination region with light pulses, wherein even when a variation inpulse energy in units of light pulses exceeds a predetermined range, anintegrated exposure value onto the photosensitive substrate (W) can becontrolled to be close to an appropriate exposure value.

A projection exposure apparatus of the present invention which comprisesa pulse light source (1) for emitting light pulses whose quantities varywithin a predetermined range for every oscillations, an illuminationsystem (2, 5-10) for radiating the light pulses from the pulse lightsource (1) onto a predetermined illumination region on a mask (R) onwhich a transfer pattern is formed, and a projection optical system (PL)for projecting an image of the pattern on the mask (R) radiated with thelight pulses into a predetermined exposure region on a photosensitivesubstrate (W), and a scanning system which synchronously scans the mask(R) and the photosensitive substrate (W) relative to the projectionoptical system upon projection of the image of the pattern, comprises:

(a) a measurement system (14-16) for detecting the intensity of thelight pulses radiated onto the photosensitive substrate (W) duringscanning of the mask (R) and the photosensitive substrate (W), andmeasuring an integrated light quantity of each of a plurality of partialregions in the exposure region on the photosensitive substrate (W) onthe basis of the detection signal;

the plurality of partial regions being defined by the scanning speed ofthe photosensitive substrate (W) and the emission interval of the lightpulses; and

(b) an adjusting system (16, 19) for adjusting the intensity of the nextlight pulse to be radiated onto the mask (R) on the basis of adifference between a target integrated light quantity and a measured.integrated light quantity of each of the plurality of partial regionsupon radiation of some light pulses onto the mask (R).

Also, an exposure apparatus according to the present invention, whichcomprises a pulse light source (1) for emitting light pulses whosequantities vary within a predetermined range for every oscillations,radiates a plurality of light pulses emitted from the pulse light source(1) onto a first object (R), synchronously scans the first object (R)and a photosensitive second object (W), and exposes a pattern on thefirst object (R) onto the second object (W), comprises:

(a) an illumination optical system (2, 5-10) for radiating the lightpulses from the pulse light source (1) onto a predetermined illuminationregion on the first object (R);

(b) a measurement system (14-16) for detecting the intensity of thelight pulses radiated onto the second object (W) during scanningexposure, and measuring an integrated light quantity on each of aplurality of partial regions in the illumination region on the secondobject, which region is irradiated with the light pulses incident on thesecond object (W) via the first object (R), on the basis of thedetection signal,

the plurality of partial regions being defined by the scanning speed ofthe second object (W) and the emission interval of the light. pulses;and

(c) an adjusting system (16, 19) for adjusting the intensity of the nextlight pulse to be radiated onto the first object (R) on the basis of adifference between the integrated light quantity and a target integratedlight quantity of each of the plurality of partial regions uponradiation of some light pulses onto the first object (R).

As described above, according to the present invention, when a patternon the mask (R) as the first object is exposed onto the photosensitivesubstrate (W) as the second object by the slit-scanning exposure methodusing light pulses from the pulse light source (1), a plurality ofpartial regions (A1, A2, A3, . . . ) on the photosensitive substrate (W)have different integrated exposure values of the radiated light pulses,as shown in, e.g., FIG. 5. Thus, the measurement system (16, 19) detectsthe intensity of light pulses radiated onto the photosensitive substrate(W), and measures the integrated exposure value so far of each of thepartial regions (A1, A2, A3, . . . ) on the basis of the detectionsignal. The adjusting system (16, 19) calculates a difference betweenthe integrated exposure value so far and a target integrated exposurevalue to be obtained upon radiation of the next light pulse for eachpartial region, and adjusts the intensity of light pulses radiated fromthe pulse light source (1) on the basis of the difference. In thismanner, an average integrated light quantity on the entire exposuresurface of the photosensitive substrate (W) can be controlled tocoincide with an appropriate exposure value within a predeterminedallowable range.

Furthermore, an exposure method according to the present invention inwhich a first object (R) is irradiated with light pulses whosequantities vary within a predetermined range for every oscillations, andthe first object (R) and a photosensitive second object (W) aresynchronously scanned, and a pattern on the first object (R) is exposedonto the second object (W), comprises the steps of:

detecting the intensity of the light pulses radiated onto the secondobject (W) during scanning exposure;

measuring an integrated light quantity on each of a plurality of partialregions which are defined on the second object (W) by the scanning speedof the second object (W) and the oscillation interval of the lightpulses, and are present within an illumination region of the lightpulses which are incident on the second object (W) via the first object(R), when some light pulses are radiated onto the first object (R); and

adjusting the intensity of the next light pulse to be radiated onto thefirst object (R) on the basis of a difference between the measuredintegrated light quantity and a target integrated light quantity of eachof the plurality of partial regions.

As described above, according to the present invention, the intensity oflight pulses radiated onto the second object (W) during scanningexposure is detected, and an integrated exposure value on each ofpartial regions (A1, A2, A3, . . . ) is measured on the basis of thedetection signal, as shown in, e.g., FIG. 5. A difference between themeasured integrated light quantity and a target integrated exposurevalue to be obtained upon radiation of the next light pulse iscalculated, and the light quantity of the light pulse radiated from thepulse light source (1) is adjusted based on the difference. Thus, anaverage integrated light quantity on the entire exposure surface of thesecond object (W) can be controlled to coincide with an appropriateexposure value within a predetermined allowable range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a projection exposure apparatus towhich an embodiment of an exposure control apparatus according to thepresent invention is applied;

FIG. 2 is a graph showing the relationship between the applied voltageand pulse energy of a pulse laser light source 1 shown in FIG. 1;

FIG. 3 is a front view showing the arrangement of a light reduction unit3 shown in FIG. 1;

FIG. 4 is a graph showing the illuminance distribution of light pulseson a wafer of the embodiment in FIG. 1;

FIG. 5 is a view showing a change in integrated exposure value onrespective pulse count integrating regions;

FIG. 6 is a graph showing a change in integrated exposure value on thefirst pulse count integrating region on the wafer of the embodiment inFIG. 1; and

FIG. 7 is a graph showing changes in integrated exposure value on aplurality of pulse count integrating regions on the wafer of theembodiment in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of an exposure control apparatus according to the presentinvention will now be described with reference to the accompanyingdrawings. In this embodiment, the present invention is applied to aprojection exposure apparatus of a slit-scanning exposure type, whichhas a pulse-oscillation type exposure light source such as an excimerlaser source as a light source.

FIG. 1 shows the projection exposure apparatus of this embodiment.Referring to FIG. 1, a pulse laser light source 1 such as an excimerlaser source is connected to a trigger controller 20 for outputtingexternal trigger pulses. The trigger controller 20 controls oscillationof the pulse laser source 1 in accordance with a command signal from amain control system 16 for controlling the operation of the overallapparatus. Light pulses emitted from the pulse laser source 1 of thisembodiment are coherent.

The exposure control apparatus of this embodiment comprises a firstlight quantity controller 18 for coarse adjustment, and a second lightquantity controller 19 for fine adjustment as light quantity controlmeans. The second light quantity controller 19 controls the integratedcharge quantity (applied voltage) for pulse emission of the pulse lasersource 1. The second light quantity controller 19 controls the appliedvoltage to the pulse laser source 1 on the basis of a command signalfrom the main control system 16, thereby finely adjusting pulse energy(exposure energy) in units of light pulses emitted from the pulse lasersource 1.

FIG. 2 shows the relationship between the applied voltage and pulseenergy. As shown in FIG. 2, by changing the applied voltage to the pulselaser source 1, the energy of light pulses emitted from the pulse lasersource 1 can be changed to have almost a linear relationship with theapplied voltage. In this embodiment, pulse energy is finely adjusted inunits of light pulses by changing the applied voltage to the pulse lasersource 1. Also, a method for finely adjusting pulse energy by changing acurrent to be supplied to the laser pulse source 1 is available.

Referring back to FIG. 1, the pulse laser source 1 is constituted as alaser source having a stable resonator, which has a narrow-bandwavelength stabilizing mechanism constituted by an etalon, a dispersionelement, or the like on a portion between two oscillation mirrors whichare arranged at two ends to sandwich a laser tube therebetween. Thepulse laser source 1 oscillates ultraviolet rays of a wavelength capableof exposing a photoresist layer on a wafer W, e.g., a KrF excimer laserbeam (wavelength=248 nm) by causing a high-voltage discharge between twoparallel electrodes extending along the optical axis of the laser beam.A laser beam LB0 emitted from the pulse laser source 1 has a rectangularsectional shape according to the arrangement pattern of these twoelectrodes, i.e., the beam section has a rectangular shape having anaspect ratio of about 1/2 to 1/5. The laser beam LB0 is incident on abeam shaping optical system 2 comprising a cylindrical lens, a beamexpander, and the like, and the beam shaping optical system 2 outputs alaser beam LB1 which is shaped to have a square beam sectional shape,and to have a size which allows efficient incidence on a fly-eye lens 5(to be described later).

The laser beam LB1 is incident on a light reduction unit 3. The lightreduction unit 3 attenuates the incident laser beam at a desired ratioby continuously or discretely changing the transmittance for theincident laser beam within a range from 100% (complete transmission) to0% (complete shielding). The transmittance of the light reduction unit 3is determined by an appropriate exposure value and the number Next ofpulses for a certain point on the wafer W required for actual exposure,which is, in turn, determined by the number N_(sp) required forsmoothing an interference pattern formed on a reticle R or a wafer W,and the number N_(e) of pulses required for controlling an integratedexposure value at a certain point on the wafer W with a desired exposurevalue control precision. This will be described in detail later.

If the transmittance of the light reduction unit 3 is set to have, e.g.,six discrete steps, the transmittance is selected before the beginningof exposure, and is not changed to another value at least duringexposure onto a single exposure field on the reticle R. In other words,the light reduction unit 3 uniformly attenuates the quantities of alllight pulses at a predetermined light reduction ratio as long as theexposure condition (an appropriate exposure value for a certain point onthe wafer W) onto the wafer W remains the same. Therefore, the lightreduction unit 3 may comprise a light quantity fine adjustment mechanismhaving a relatively low response speed (switching speed betweendifferent transmittances). The light reduction unit 3 of this embodimentadopts a mechanism which comprises six mesh filters attached to arevolver, and having different transmittances, and rotates the revolver.

FIG. 3 shows the revolver type light reduction unit 3. Referring to FIG.3, six different mesh filters 30a to 30f are attached to a disk-shapedrevolver plate 30 at angular intervals of about 60° to have therotational axis as the center. One of the mesh filters 30a to 30f is setin the optical path of the laser beam LB1 shown in FIG. 1. In FIG. 3,the transmittance of the mesh filter 30a is about 100%, and thetransmittances of the remaining mesh filters 30b to 30f are set togradually decrease.

As light reduction elements to be attached to the revolver plate 30,dielectric mirrors having different transmittances may be used in placeof the mesh filters. Also, when two sets of revolver plates 30 arearranged at a predetermined interval to be rotated relative to eachother, the transmittances of the first revolver plate are set to be,e.g., 100%, 90%, 80%, 70%, 60%, and 50%, and those of the secondrevolver plate are set to be, e.g., 100%, 40%, 30%, 20%, 10%, and 5%, atotal of 36 different transmittances can be realized by combining thesetwo plates.

As a light reduction method of the light reduction unit 3, variousmethods can be used. For example, a diaphragm having a predeterminedrectangular aperture can be combined with a zoom lens system, and lightreduction can be continuously performed by changing the combination ofthe variable zoom ratio and the variable width of the rectangularaperture. Furthermore, a method of rotating a so-called etalon obtainedby holding two glass plates (quartz plates or the like) to besubstantially parallel to each other, or a method of moving two phasegratings or amplitude gratings relative to each other may be used.Alternatively, when a linearly polarized laser beam is used as exposurelight, a method of rotating a polarization plate may be adopted as thelight reduction method of the light reduction unit 3.

Referring back to FIG. 1, a substantially collimated laser beam LB1'which is attenuated by the light reduction unit 3 at a predeterminedlight reduction ratio is incident on the fly-eye lens (opticalintegator) 5 via an interference fringe reduction unit 4 for averagingan interference pattern. The interference fringe reduction unit 4 has avibration mirror which is one-dimensionally (or two-dimensionally)vibrated by an actuator (e.g., a piezo element), and one-dimensionally(or two-dimensionally) moves an interference pattern on the reticle R(or the wafer W) by changing the incident angle of the laser beam LB1'incident on the fly-eye lens 5 in units of light pulses, thereby finallysmoothing the interference pattern. In other words, the interferencefringe reduction unit 4 is used for increasing the uniformity of theilluminance of a pulse laser beam on the reticle R (or the wafer W), andthe details of its principle are disclosed in U.S. Pat. No. 4,619,508.

The interference fringe reduction unit 4 may comprise an arrangement forrotating, e.g., a diffusion plate in synchronism with emission of lightpulses in place of the arrangement using the vibration mirror.

A laser beam IL2 emerging from the fly-eye lens 5 is incident on a beamsplitter 6 having a high transmittance and a low reflectance. The laserbeam IL2 transmitted through the beam splitter 6 is incident on a fieldstop 8 via a first relay lens 7A. The sectional shape of the laser beamIL2 is shaped into a slit shape by the field stop 8. The arrangementplane of the field stop 8 is located at a position conjugate with thepattern formation surface of the reticle R and the exposure surface ofthe wafer W, and by adjusting the shape of the aperture portion of thefield stop 8, an illumination field having a desired shape can beobtained on the reticle R. The laser beam IL2 emerging from the apertureportion of the field stop 8 illuminates a portion of a pattern region onthe reticle R with a slit-shaped illumination region 25 via a secondrelay lens 7B, a bending mirror 9, and a main condenser lens 10. Thereticle R is placed on a reticle stage 11.

A laser beam diffracted by the pattern region on the reticle R forms apattern image on the reticle R onto a photoresist layer as aphotosensitive material on the wafer W via a projection optical systemPL. More specifically, an image of a circuit pattern in the slit-shapedillumination region 25 is projected onto the exposure surface of thewafer W in a slit-shaped exposure region 26 conjugate with theslit-shaped illumination region 25 on the reticle R. The wafer W isvacuum-chucked on a wafer holder 12 on a wafer stage 13, and the waferstage 13 is constituted by an X stage for scanning the wafer W in the Xdirection as one direction in a plane perpendicular to the optical axisof the projection optical system PL, a Y stage for aligning the wafer Win the Y direction perpendicular to the X direction in the planeperpendicular to the optical axis, a Z stage for aligning the wafer W inthe Z direction parallel to the direction of the optical axis, and thelike.

Upon execution of exposure based on a slit-scanning exposure method, areticle stage scanning control system 21 and a wafer stage scanningcontrol system 22 respectively drive the reticle stage 11 and the waferstage 13 on the basis of commands from the main control system(controller) 16. The wafer W is scanned in the -X direction with respectto the slit-shaped exposure region 26 in synchronism with scanning ofthe reticle R in the X direction with respect to the slit-shapedillumination region 25. The relationship among an appropriate exposurevalue, the scanning speed in synchronous scanning, the laser oscillationfrequency, and the like will be described in detail later.

Of the laser beam IL2 emerging from the fly-eye lens 5, a laser beamreflected by the beam splitter 6 is focused on the light-receivingsurface of a light-receiving element (photelectric detector) 15 by acondensing optical system 14. The light-receiving element 15 preciselyoutputs a photoelectric signal according to the light quantity (lightintensity) of each light pulse of the laser beam, and comprises a PINphotodiode having a sufficient sensitivity in an ultraviolet region, andthe like. A photoelectric signal output from the light-receiving element15 is supplied to the main control system 16, and the main controlsystem 16 include a calculator which sequentially integrates the lightquantities of light pulses.

The measured value (integrated light quantity) serves as fundamentaldata upon control of the applied voltage in units of light pulses forthe pulse laser source 1 and upon execution of oscillation, control inunits of light pulses of the pulse laser source 1 via the triggercontroller 20 in the main control system 16. Note that the relationshipbetween the illuminance of the laser beam on the exposure surface of thewafer W and the photoelectric signal output from the light-receivingelement 15 is obtained by, e.g., a power meter in advance, and is storedin a memory 23.

The main control system 16 is connected to an input-output device 24 andthe memory 23. On the basis of the measured value from thelight-receiving element 15, the main control system 16 outputs a controlcommand to the trigger controller 20 and also outputs predeterminedcommand signals to the first and second light quantity controllers 18and 19, and an interference fringe reduction controller 17. The maincontrol system 16 systematically controls the operation 10 of the entireprojection exposure apparatus. The input-output device 24 serves as aman-machine interface between an operator and the projection apparatusmain body, transmits various parameters necessary for exposure from theoperator to the main control system 16, and informs the operation stateof the main control system 16 to the operator.

The memory 23 stores parameters (constants) and tables input from theinput-output device 24 and required for an exposure operation, variouscalculations, and the like; photosensitive characteristics of thelight-receiving element 15; and the like. In particular, in thisembodiment, the memory 23 stores information of a minimum number N_(sp)of pulses required for satisfactorily smoothing an interference patternby the interference fringe reduction unit 4, and the number N_(e) ofpulses required for controlling an integrated exposure value with adesired exposure value control precision.

A method of determining a transmittance α of the reduction unit 3 and asynchronous scanning speed v (cm/sec) of the wafer stage 13 by the maincontrol system 16 will be described below. If the photoresistsensitivity on the wafer W is represented by S (mj/cm²), the energydensity per light pulse on the exposure surface of the wafer W in anon-light reduction state is represented by p (mj/cm² ·pulse), thetransmittance of the first light quantity controller 18 is representedby α, the transmittance of the second light quantity controller 19 isrepresented by β, the slit width, in the scanning direction, of theslit-shaped exposure region 26 on the exposure surface of the wafer W isrepresented by D (cm), and the laser oscillation frequency of the pulselaser source 1 is represented by f (Hz), the number N_(exp) of pulsesrequired for exposing a certain point on the exposure surface of thewafer W is given by:

    N.sub.exp =S/(α·βp)=d·f/v=integer(1)

From formula (1), S/(α·β·p) must be converted into an integer, andconversely, when S/(α·βp) cannot be converted into an integer even afterthe transmittance β is finely adjusted, an offset (error) from a targetvalue undesirably results upon exposure. Therefore, the transmittance βmust be largely changed by a method of uniformly controlling all lightpulses in place of a method of controlling each light pulse by thesecond light quantity controller 19. Similarly, D·f/v in formula (1)must be converted into an integer. When the slit width D in the scanningdirection is constant, and the laser oscillation frequency f assumes amaximum value (such a case is advantageous in terms of the throughput),the scanning speed v must be adjusted.

When the photoresist has a low sensitivity, i.e., the sensitivity S hasa large value, it is preferable that the scanning speed v be decreased.When the photoresist has a high sensitivity, i.e., the sensitivity S hasa small value, the scanning speed v must be increased. However, thescanning speed v has an allowable maximum value v_(max). For thisreason, when the scanning speed v exceeds its maximum value, thetransmittance α must be decreased by controlling the light reductionunit 3 by the first light quantity controller 18, so that the scanningspeed v becomes smaller than the maximum value v_(max). The numberN_(exp) of pulses must be larger than the minimum number N_(sp) ofpulses required for smoothing an interference pattern, and the numberN_(e) of pulses required for controlling an integrated exposure valuewith a desired exposure value control precision. To summarize theabove-mentioned conditions, we have:

    v.sub.max ≧(D·f/s)·α·βp(2)

    N.sub.exp =S/(α·β·p)≧Max (N.sub.e, N.sub.sp)                                                 (3)

where the function Max(A, B) indicates a larger one of values A and B.From formulas (1) and (3), the following formulas are established:##EQU1## where the function Min(A, B) indicates a smaller one of valuesA and B. When the transmittance α must be set, the transmittance β isre-set based on formula (1) after the transmittance α is set.Thereafter, the scanning speed v is determined based on formula (1).

Energy fine adjustment executed when energy is adjusted via the secondlight quantity controller 19 in units of light pulses emitted from thepulse laser source 1 will be described below.

From formula (1), a distance X_(step) by which the wafer stage 13 isscanned in the -X direction as the scanning direction during each pulseemission interval of the pulse laser source 1 is given by:

    X.sub.step =D.N.sub.exp                                    (5)

In order to rewrite this formula, if the exposure surface of the wafer Wis divided into regions each having the width X_(step) in the scanningdirection (to be referred to as "pulse count integrating regions"hereinafter), the width D, in the scanning direction, of the slit-shapedexposure region 26 on the wafer W is defined by multiplying N_(exp)equal to the number of exposure pulses with the width X_(step), in thescanning direction, of each of the pulse count integrating regions.

The integrated exposure value (light quantity) on each of a plurality ofpulse count integrating regions on the wafer W will be described belowwith reference to FIGS. 4 and 5.

FIG. 4 shows a state wherein the exposure surface of the wafer W isdivided into the pulse count integrating regions. In FIG. 4, the Xcoordinate at a certain timing on the wafer W is plotted along theabscissa, and an illuminance IW at each X position is plotted along theordinate. FIG. 4 shows a case wherein the width D of the slit-shapedexposure region is four times the width X_(step), in the scanningdirection, of the pulse count integrating region, i.e., the numberN_(exp) of exposure pulses is four (in practice, several 10 pulses ormore are required). In FIG. 4, the slit-shaped exposure region scans thewafer W in the X direction for the sake of simplicity. When theilluminance distribution by the first light pulse is represented by arectangular illuminance distribution 26A, an illuminance distribution26B by the second light pulse is shifted by X_(step) in the X directionfrom the illuminance distribution 26A by scanning of the wafer Wrelative to the slit-shaped exposure region. Similarly, an illuminancedistribution 26C by the third light pulse is shifted by X_(step) fromthe illuminance distribution 26B in the X direction. Then, theilluminance distributions by the light pulses are successively shiftedby X_(step) in the X direction. The value of the illuminancedistribution IW by energy of each light pulse varies due to a variationin output from the pulse laser source 1.

For this reason, a pulse count integrating region A1 of the widthX_(step) irradiated with the first light pulse, a pulse countintegrating region A2 of the width X_(step) irradiated with the secondlight pulse, and a pulse count integrating region A3 of the widthX_(step) irradiated with the third light pulse have different integratedexposure values, respectively.

A method of calculating energy of the next pulse laser beam to beradiated from the pulse laser source 1 in the main control system 16will be described below with reference to FIG. 5. FIG. 5 shows a changein integrated exposure value over time on each pulse count integratingregion. Also, FIG. 5 shows a state wherein the wafer W is scanned in the-X direction relative to the slit-shaped exposure region, and the pulselaser source 1 emits a light pulse.. Assume that the transmittance α isdetermined in advance by formula (4), and the transmittance β is finelyadjusted to convert (N_(exp) =S/ (α·β·p)) in formula (1) into aninteger. The pulse energy density on the exposure surface of the wafer Wis represented by q (mJ/cm² ·pulse) (=α·β·pa). A variable pa is anaverage value of the energy density p per pulse on the exposure surfaceof the wafer W in a non-light reduction state.

When the first pulse light is emitted to the wafer W, the main controlsystem 16 adjusts the applied voltage of the pulse laser source 1 viathe second light quantity controller 19 by the above-mentionedcalculation, so that the light pulse has energy q. In this case,assuming that the exposure value of the first pulse actually detected bythe light-receiving element 15 is e1, as shown in FIG. 5, the maincontrol system 16 calculates a difference D11 between the integratedexposure value e1 by the first pulse and a target integrated exposurevalue (target exposure dose) 2q by the second pulse on the pulse countintegrating region A1. Furthermore, the main control system 16calculates a difference D21 (=q) between an integrated exposure value 0by the first pulse and the target integrated exposure value q on thepulse count integrating region A2. Then, the control system 16calculates an average value (D11+D21)/2 of the calculated differences.The main control system 16 finely adjusts the applied voltage of thepulse laser source via the second light quantity controller 19, so thatthe light quantity of the second pulse emitted from the pulse lasersource 1 becomes equal to the average value (D11+D21)/2 of thedifferences. Thereafter, the second pulse is emitted. If the actuallydetected exposure value of the second pulse is e2, the main controlsystem 16 calculates a difference D12 between the integrated exposurevalue (e1+e2) and a target integrated exposure value 3q by the thirdpulse on the pulse count integrating region A1. Similarly, the controlsystem 16 calculates a difference D22 between the integrated exposurevalue and a target integrated exposure value 2q by the third pulse onthe pulse count integrating region A2, and a difference D32 between theintegrated exposure value and the target integrated exposure value q bythe third pulse on the pulse count integrating region A3, and thencalculates an average value (D12+D22+D32)/3 of these differences. Themain control system 16 finely adjusts the applied voltage of the pulselaser source 1 via the second light quantity controller 19, so that thelight quantity of the third pulse becomes equal to (D12+D22+D32)/3, andthereafter, the third pulse is emitted. Similarly, upon completion ofemission of the third pulse, the main control system 16 calculatesdifferences D13, D23, D33, and D43 between the integrated exposurevalues by the third pulse and the target integrated exposure values bythe fourth pulse on the pulse count integrating regions A1, A2, A3, andA4, and adjusts the light quantity of the light pulse on the basis of anaverage value of these differences. At the fifth pulse as well, the maincontrol system 16 calculates differences D24, D34, D44, and D54 betweenintegrated exposure values by the fourth pulse and the target integratedexposure values by the fifth pulse on the pulse count integratingregions A2, A3, A4, and A5, and adjusts the light quantity of the lightpulse on the basis of an average value of these differences. If thefirst pulse with which the exposure surface of the wafer W is actuallyexposed by scanning exposure is represented by n=1, a target lightquantity Q_(n) of an n-th pulse can be obtained by the followingformulas: ##EQU2## where Me=L/X_(step), and L is the length, in thescanning direction, of an exposure field on the wafer W. The term i·q ineach of the second, third, and fourth formulas in formulas (6)represents the target integrated exposure value on the i-th pulse countintegrating region, and the term ΣTj therein represents the integratedexposure value of light pulses exposed so far on the i-th pulse countintegrating region. Therefore, each of the second, third, and fourthformulas in formula (6) means that the average value of differencesbetween integrated exposure values so far on all pulse count integratingregions having the number of exposure pulses>N_(exp), and the targetintegrated exposure values by the next pulse is defined as energy of thenext pulse emission of the pulse laser source 1. More specifically, themain control system 16 in FIG. 1 adjusts the applied voltage of thepulse laser source 1 via the second light quantity controller 19, sothat the average value of pulse energy to be radiated by the next lightpulse on the pulse count integrating regions (A1, A2, A3, . . . in FIG.4) is used as pulse energy of the next pulse emission of the pulse lasersource 1.

The control state of the integrated exposure value on the pulse countintegrating region A1 in FIG. 5 will be described below with referenceto FIG. 6. FIG. 6 shows a change in integrated exposure value in unitsof pulse emissions on the pulse count integrating region A1. In FIG. 6,a solid polygonal line represents the actually integrated exposurevalue, and an alternate long and two short dashed line represents thetarget integrated exposure value at the timing of radiation of eachlight pulse. FIG. 6 shows a case wherein the number N_(exp) of exposurepulses is 8, i.e., the width D, in the scanning direction, of theslit-shaped exposure region 26 is 8·X_(step) for the sake of simplicity.If the actual exposure value for a target integrated exposure value P₁of the first pulse is represented by P₁, an exposure value P₂ as adifference between a target integrated exposure value (=2·P₁) of thesecond pulse and the actual exposure value P₁ ' is energy to be radiatedby the second pulse on the pulse count integrating region A1.

In this embodiment, in place of directly using the exposure value P₂,differences between the target integrated exposure values by the secondpulse and the integrated exposure values so far are calculated for thepulse count integrating regions to be exposed by the next pulseemission, and an average value of these differences is defined as energyto be radiated by the second pulse. The applied voltage of the pulselaser source 1 in FIG. 1 is controlled to obtain this energy. As aresult, on the pulse count integrating region A1, energy of, e.g., anexposure value P₂ ' is radiated upon radiation of the second lightpulse. As for the third pulse and subsequent pulses, differences betweentarget integrated exposure values and actual integrated exposure valuesare calculated for the pulse count integrating region A1 and other pulsecount integrating regions, and an average value of these differences isdefined as an exposure value of the next light pulse. The details of amethod of calculating a target value of exposure energy by next pulseemission on each pulse count integrating region are filed as, e.g., U.S.Ser. No. 623,176 (Dec. 12, 1990).

When exposure is performed in this manner, the main control system 16 inFIG. 1 calculates the target value of pulse energy by the next lightpulse in units of light pulses using formulas (6), and executes lightquantity control of the pulse laser source 1 via the second lightquantity controller 19 to obtain the calculated pulse energy. Theexposure value control precision A of the method of this embodiment isgiven by: ##EQU3## where (Δp/pa) is the variation in pulse energy.

In order to obtain an exposure value control precision A=1% at(Δp/pa)=10%, N_(exp) ≧50 pulses need only be satisfied from formula (7).In other words, N_(e) ≃50. The number N_(sp) of pulses in formula (3)required for reducing an interference fringe is normally experimentallydetermined, and 50 pulses are reckoned to suffice. Therefore, Max(N_(e),N_(sp))=50 need only be set in formula (3).

In this embodiment, light quantity adjustment of light pulses isperformed for each pulse. Alternatively, if a target light quantity Qnof the n-th pulse calculated in formulas (6) above falls within a lightquantity range (adjustable range) of a light pulse which can be adjustedby the second light quantity controller 19, light quantity adjustmentcan be performed in units of an arbitrary number of pulses (e.g., inunits of five pulses). The second light quantity controller 19 may becontrolled to change the light quantity adjustment interval duringsingle scanning exposure (e.g., adjustment in units of five pulses maybe changed to adjustment in units of three pulses during exposure).Furthermore, only when the target light quantity Qn of the n-th pulseexceeds a predetermined allowable range in the above-mentionedadjustable range, the second light quantity controller 19 may becontrolled to perform light quantity adjustment.

In this embodiment, upon radiation of the n-th light pulse, a differencebetween the integrated exposure value at that time and the targetintegrated exposure value to be given upon radiation of the (n+1)-thpulse is calculated. Alternatively, a difference (error) between theintegrated exposure value and the target integrated exposure value uponradiation of the n-th light pulse may be calculated. Thus, an averagevalue of errors obtained by the respective pulse count integratingregions is used as an offset in light quantity adjustment of the(n+1)-th light pulse, thereby adjusting the light quantity of the lightpulse via the second light quantity controller 19.

Furthermore, in this embodiment, an average value of differences betweenthe integrated exposure values and the target integrated exposure valueson a plurality of pulse count integrating regions is calculated, and theaverage value is set to be a light quantity of the next light pulse.However, the present invention is not limited to this. For example,maximum and minimum values of these differences may be used, and anaverage value of the maximum and minimum values may be set to be a lightquantity of the next light pulse.

Finally, in the case of the slit-scanning exposure type exposureapparatus, since pulses radiated in units of pulse count integratingregions in an exposure field are slightly shifted from each other, theabove-mentioned exposure value control precision A varies in units ofpulse count integrating regions. For example, assuming that N_(exp)pulses from the n-th pulse to the (n+N_(exp) -1)-th pulse are radiatedon a certain pulse count integrating region, N_(exp) pulses from the(n+1)-th pulse to the (n+N_(exp))-th pulse are radiated on a neighboringpulse count integrating region on the trailing side in the scanningdirection. For this reason, the main control system 16 discriminates theexposure value control precision in units of pulse count integratingregions. In this case, an internal memory for the integrated lightquantity in the main control system 16 must have at least a capacitycorresponding to N_(exp) pulses, and ideally, it is desirable to preparea capacity corresponding to L/X_(step).

A method of discriminating whether or not exposure with an appropriateexposure value is performed on a wafer in a case wherein exposure isperformed in units of pulse count integrating regions, as indicated bypolygonal lines in FIG. 6, will be explained below with reference toFIG. 7. In FIG. 7, a polygonal line 28A represents a change in exposurevalue in the pulse count integrating region A1 shown in FIG. 6, andother polygonal lines 28B to 28E respectively represent changes inexposure values in the pulse count integrating regions A2 to A8 (notshown). In this case, in the first pulse count integrating region A1, adifference ΔE_(A) between an appropriate exposure value E_(ade) and anactually integrated exposure value is calculated upon completion ofexposure of the last pulse. When the difference ΔE_(A) exceeds apredetermined allowable value, the main control system 16 determinesthat the exposure value onto the wafer is not appropriate, andterminates the exposure process onto the wafer in an abnormal exposurestate.

When the difference ΔE_(A) is equal to or smaller than the predeterminedallowable value, a difference between the appropriate exposure valueE_(ade) and the actually integrated exposure value for each of the pulsecount integrating regions A2, A3, . . . is calculated, and it is checkedif differences ΔE_(B), ΔE_(C), . . . exceed the predetermined allowablevalue. Then, as indicated by the polygonal line 28E in FIG. 7, when adifference ΔE_(E) between the appropriate exposure value E_(ade) and theactually integrated exposure value on the pulse count integrating regionA8 (not shown) exceeds the predetermined allowable value, the exposureprocess onto the wafer is terminated in an abnormal exposure state. Whenthe difference ΔE_(E) is equal to or smaller than the predeterminedallowable value, a difference between the appropriate exposure valueE_(ade) and the actually integrated exposure value on the next pulsecount integrating region is similarly calculated, and it is checked ifthe calculated difference exceeds the predetermined allowable value.Thus, it can be quickly and precisely discriminated whether or not theexposure value onto the wafer is appropriate.

The above-mentioned discrimination may be made not only when N_(exp)pulses are radiated onto each pulse count integrating region but alsowhen an arbitrary number of light pulses are radiated. Morespecifically, when the n-th light pulse is radiated onto an exposurefield, the main control system 16 calculates differences between theintegrated exposure values and the target integrated exposure values atthat time in units of pulse count integrating regions, and if the system16 detects any difference which exceeds the predetermined allowablevalue, it may terminate the exposure process onto the wafer at thattime.

In this embodiment, as a method of adjusting the energy of each lightpulse during single scanning exposure, a method of controlling theapplied voltage to the pulse laser source 1 is used. Various othermethods may be used as long as they can obtain a transmittance whichchanges continuously, and have a high response speed. More specifically,a combination of an aperture and a zoom lens system, an etalon, twophase gratings or density gratings, a rotary polarization plate (in thecase of a linearly polarized light laser), or the like, which have beendescribed above as examples of the light reduction unit 3, may be used.

As described above, the present invention is not limited to theabove-mentioned embodiment, and various changes and modifications may bemade within the spirit and scope of the invention.

What is claimed is:
 1. A scanning exposure apparatus comprising:anillumination system for illuminating a mask with light pulses; aprojection optical system receiving light pulses from said mask andhaving an image field which is smaller than an exposure region on aphotosensitive substrate, in which region an image of a pattern on saidmask is to be formed by light pulses radiated from said projectionoptical system; a scanning system which synchronously scans said maskand said substrate relative to said projection optical system forscanning exposure; a measurement system which measures, during thescanning exposure, individual integrated light quantities radiated oneach of a plurality of partial regions, respectively in said exposureregion, within the image field to be irradiated with a next light pulse,based on intensities of light pulses radiated on said substrate; and anadjusting system which adjusts an intensity of said next light pulsebased on the measured integrated light quantities.
 2. An apparatusaccording to claim 1, wherein said illumination system has a lightsource which radiates said light pulses, and said adjusting systemincludes a device which adjusts a voltage applied to said light sourceto vary intensity of the light pulses.
 3. An apparatus according toclaim 1, wherein said measurement system includes a photoelectricdetector for receiving a portion of each light pulse and a calculatorfor calculating individual integrated light quantities radiated each ofsaid plurality of partial regions, respectively based on a signal fromsaid photoelectric detector and for determining the intensity of saidnext pulse based on the calculated integrated light quantities.
 4. Anapparatus according to claim 3, wherein said illumination systemincludes an optical integrator for reducing non-uniformity inilluminance of each light pulse on said mask and an optical member forguiding a portion of each light pulse from said optical integrator tosaid photoelectric detector.
 5. An apparatus according to claim 1,further comprising a controller which detects a difference between atleast one of said integrated light quantities and a target exposure dosecorresponding thereto, the controller suspending said scanning exposurewhen said difference exceeds a predetermined value.
 6. A scanningexposure apparatus comprising:an illumination system for illuminating amask with light pulses; a projection optical system receiving lightpulses from said mask and having an image field which is smaller than anexposure region on a photosensitive substrate, in which region an imageof a pattern on said mask is to be formed by light pulses radiated fromsaid projection optical system; a scanning system which synchronouslyscans said mask and said substrate relative to said projection opticalsystem for scanning exposure; and a measurement system which measures,during the scanning exposure, individual integrated light quantitiesradiated on each of a plurality of partial regions, respectively, insaid exposure region, within the image field to be irradiated with anext light pulse, based on intensities of light pulses radiated on saidsubstrate; wherein an intensity of said next light pulse is determinedbased on the measured integrated light quantities.
 7. A scanningexposure apparatus comprising:an illumination system for illuminating amask with light pulses; a scanning system which synchronously scans saidmask and a substrate for scanning exposure; and a measurement system,including a photoelectric detector which receives a portion of eachlight pulse and which measures, each time a predetermined number ofpulses are radiated on the mask during the scanning exposure, individualintegrated light quantities radiated on each of a plurality of partialregions in an exposure region, respectively, on the substrate on whichan image of a pattern on the mask is formed by light pulses from themask, based on a signal from said photoelectric detector.
 8. Anapparatus according to claim 7, further comprising an adjusting systemfor adjusting, during the scanning exposure, intensity of a light pulsebased on the measured integrated light quantities.
 9. An apparatusaccording to claim 8, wherein said illumination system has a lightsource which radiates said light pulses, and said adjusting systemincludes a device which adjusts a voltage applied to said light sourceto vary intensity of a light pulse.
 10. An apparatus according to claim7, further comprising a projection optical system receiving light pulsesfrom the mask for projecting the image of the pattern of the mask ontosaid substrate, the projection optical system having a rectangular imagefield which is smaller than a size of said exposure region.
 11. Anapparatus according to claim 7, further comprising a controller whichdetects a difference between at least one of the measured integratedlight quantities and a target exposure dose corresponding thereto, thecontroller suspending said scanning exposure when said differenceexceeds a predetermined value.
 12. A scanning exposure apparatuscomprising:an illumination system for irradiating a mask with lightpulses; a scanning system which synchronously scans the mask and aphotosensitive substrate for scanning exposure, so that an image of apattern on the mask is formed by light pulses from the mask on aplurality of small regions in an exposure region on the photosensitivesubstrate during the scanning exposure, wherein, during said scanningexposure individual integrated light quantities radiated on each of saidplurality of small regions vary among said plurality of small regions;and a measurement system which measures, each time a predeterminednumber of pulses are radiated on the mask during the scanning exposure,each of the individual integrated light quantities radiated on saidplurality of small regions, respectively.
 13. A scanning exposureapparatuscomprising: an illumination system for illuminating a mask withlight pulses; a projection optical system receiving light pulses fromsaid mask and having an image field which is smaller than a size of anexposure region on a photosensitive substrate, in which region an imageof a pattern on said mask is to be formed by light pulses radiated fromsaid projection optical system; a scanning system which synchronouslyscans said mask and said substrate relative to said projection opticalsystem for scanning exposure, wherein, during said scanning exposure,individual integrated light quantities radiated on each of a pluralityif small regions in said exposure region within said image field varyamong said plurality of small regions; and an adjusting system whichadjusts intensity of said light pulses in accordance with the individualintegrated light quantities radiated on each of the plurality of smallregions, respectively.
 14. A method for scanningly exposing an exposureregion on a photosensitive substrate with an image of a pattern on amask, utilizing light pulses irradiating the mask and projected from themask to the substrate by a projection optical system, the exposureregion being larger than an image field of the projection opticalsystem, comprising the steps of:synchronously scanning said mask andsaid substrate relative to said projection optical system for scanningexposure, wherein, during said scanning exposure, individual integratedlight quantities radiated on each of a plurality of small regions issaid exposure region within said image field vary among said pluralityof small regions; measuring, each time a predetermined number of pulsesare radiated on the mask during the scanning exposure, each of theindividual integrated light quantities radiated on said plurality ofsmall regions, respectively; and adjusting an energy amount of a nextlight pulse to be radiated on the substrate based on measured integratedlight quantities.
 15. A method according to claim 14, wherein a voltageapplied to a light source which radiates aid light pulses is adjusted sothat an energy amount of a light pulse is varied.
 16. A method accordingto claim 14, further comprising the step of:suspending said scanningexposure, when a difference between at least one of said integratedlight quantities and a target exposure dose corresponding theretoexceeds a predetermined value.
 17. A scanning exposure method comprisingthe steps of:synchronously scanning a mask and a substrate relative to aprojection optical system so to scanningly expose an exposure region onthe substrate with an image of a pattern on a mask, utilizing lightpulses irradiating the mask, the exposure region being larger than animage field of the projection optical system, wherein, during a scanningexposure, individual integrated light quantities radiated on a pluralityof small regions in said exposure region within said image field varyamong said plurality of small regions; and measuring, each time apredetermined number of pulses are radiated on the mask during thescanning exposure, each of the individual integrated light quantitiesradiated on said plurality of small regions, respectively.
 18. A methodaccording to claim 17, further comprising the step of:adjusting anenergy amount of a next light pulse to be radiated onto said substrate,when a difference between at least one of said integrated lightquantities and a target integrated light quantity corresponding theretoexceeds a predetermined value.
 19. A method according to claim 17,further comprising the step of:suspending said scanning exposure, when adifference between at least one of said integrated light quantities anda target exposure dose corresponding thereto exceeds a predeterminedvalue.
 20. A method according to claim 17, further comprising the stepof:determining intensity of a next light pulse for irradiating said maskin accordance with differences between the individual measuredintegrated light radiated quantities on each of said plurality of smallregions and target integrated light quantities respectivelycorresponding thereto.
 21. A scanning exposure method comprising thesteps of:synchronously scanning a mask and a substrate relative to aprojection optical system so as to scanningly expose an exposure regionon the substrate with an image of a pattern on the mask, utilizing lightpulses irradiating the mask during a scanning exposure, the exposureregion being larger than an image field of the projection opticalsystem,; and adjusting an energy amount of a next light pulse to beradiated onto said substrate in accordance with individual integratedlight quantities radiated on each of a plurality of small regions,respectively, in said exposure region within said image field during thescanning exposure.